The invention relates to a belt tensioning device for a belt drive which comprises an infinite belt and at least two belt pulleys of which one can function as an input and one as an output. Such belt drives are used more particularly in connection with internal combustion engines of motor vehicles for driving auxiliary apparatuses or accessories, wherein a first belt pulley is positioned on the crankshaft of the internal combustion engine and drives the belt. Further, belt pulleys are associated with the auxiliary apparatuses, such as a water pump, a generator, or an air conditioning compressor and are rotatingly driven by the belt. In conventional belt drives, the auxiliary apparatuses are designed as consumers, i.e., they are driven by the belt pulley of the crankshaft via the belt. A slack side of the belt is formed between the crankshaft and the apparatus adjoining the crankshaft in the direction of circulation of the belt, the adjoining apparatus typically being a generator. In order to ensure that the belt is sufficiently wrapped around the belt pulley, the belt is pretensioned by a tensioning roller of the belt tensioning device.
There are also prior art belt drives which comprise an auxiliary apparatus in the form of a starter generator, i.e., an electric motor which, depending on the operating condition, can be operated as a starter or as a generator. Under normal or engine operating conditions, the belt pulley at the crankshaft is the driving pulley, whereas the starter generator as well as the remaining apparatuses are driven. Under starter operating conditions, the starter generator drives the crankshaft via the associated belt pulley in order to start the internal combustion engine. In such belt drives having a starter generator as an auxiliary apparatus, between the engine operation on the one hand and the starter operation on the other hand, a change takes place between the driving side of the belt and the slack side of the belt on both sides of the belt pulley of the starter generator. It is therefore necessary to provide spring-loaded tensioning rollers for both sides, of which spring-loaded tensioning rollers one is effective under spring load at the slack side of the belt, whereas the other one is pushed back from the tensioned driving side of the belt.
From EP 0 858 563, there is known a tensioning device with a fixed tensioning arm axle. The tensioning arm axle is provided with a supporting flange, which is meant to contact an engine block. On one end of the tensioning arm axle, a tensioning arm is rotatably supported by means of a conical friction bearing. A coaxially arranged helical torsion spring is clamped in between the supporting flange and a support fixed at the tensioning arm, for the purpose of applying an axial pressure force and a torsional force.
Document DE 10 2004 047 422 A1 proposes a belt tensioning device with a receiving housing having a hub coaxially arranged therein and a tensioning arm. The receiving housing and the hub are rotatably supported relative to one another with pre-tension by means of a helical torsion spring. There is provided a damping device which comprises a circumferentially slotted damping bush and a band spring adapted to the damping bush.
From EP 2 128 489 A2, there is known a belt tensioning device for a belt drive with a starter generator. The belt tensioning device comprises a housing in which two tensioning arms are pivotably supported around a joint pivot axis. The tensioning arms are supported relative to one another by spring means. With the driving belt pulley mounted at the starter generator, the housing can be mounted contact-free relative to the starter generator in an annular region surrounding the driveshaft of the starter generator.
U.S. Pat. No. 4,758,208 proposes a belt tensioning device for a belt drive with a starter generator. The housing of the starter generator is fixed at the engine block such that the housing is rotatable to a limited extent around the axis of rotation of the driving pulley. The belt tensioning device comprises two tensioning arms which, scissor-like, are pivotable relative to one another around the driveshaft. Between the tensioning arms, there is arranged a tensioning spring that loads the two tensioning arms towards one another.
It is an object of the present invention to propose a belt tensioning device for a belt drive, which belt tensioning device is compact in design and independent of the construction and space conditions of the main drive. Furthermore, an object consists in proposing an apparatus with such a belt tensioning device that requires only a small amount of space.